Exposure TimesWhen attached to a telescope the f0.5 focal reducer will typically reduce exposure times to near a quarter when imaging extended objects like galaxies, nebulae and the Moon.

I'm just wondering if I'm missing something obvious. From theory elsewhere, I understand that focal reducers only work at their magnification factor when placed at a set distance from the focal plane. At the moment I suspect I'm not at that distance, but I don't know what it would be.

They may mean CCD imaging, with a DSLR sized sensor and a mirror box this might be the result. I don't see how a 1.25" lens could cover such a large sensor. This is way outside my knowledge of optics though...

I've tried to find some kind of focal reducer/corrector for the 4SE but can't find any. Bah....

Did you use a 2inch tube spacer after the focal reducer ? I have use my 0.63 focal reducer on my C8 plenty of time and never experience any of theses issue. the FR does 2 thing : Reduce Focal Lenght and flattend the field a little. I think you might experience coma on the edge of the image 0.5 FR.

I'm guessing the FR on C8 is the official matched one? Here I'm using a generic. My guess is still that the reducer is at the "wrong" distance from the sensor so it is not giving the expected effect, but I've been too lazy to investigate further.

I'm guessing the FR on C8 is the official matched one? Here I'm using a generic. My guess is still that the reducer is at the "wrong" distance from the sensor so it is not giving the expected effect, but I've been too lazy to investigate further.

I use an Antares FR 6.3. I always believe that FR 5.0 are too aggressive. The picture is my C8 is always inverted without or without the FR. You have a Mak right ? you may want to make sure that your FR is compatible with Mak.

Ohh, it isn't a flattener. That's the reason for the distortion then I guess...

After doing some searching on the 'net, I found that the distance from corner to corner on an APS-C sized sensor is about 30.1mm, while your telescope has a baffle tube that's 1.25" (31.75mm) across - This means the sensor just avoids being clipped. With a focal reducer, that size goes down as it 'compresses' the light cone leaving you with an image plane that's too small for the sensor. Now the clipping finally makes sense to me...

With that said, the results I'm seeing in the images you posted make sense. Annoyingly, this means that you'll end up with clipping no matter what focal reducer you use. Only option is to get a scope with a bigger baffle tube.

Ok, 2 and a half years since I started this post, I finally bothered to figure out the generic focal reducer I picked up then.

Generically, it helped me understand the relationship of the focal reducer and its positioning. In normal use, the magnification of a reducer varies from 1 when placed on the focal plane, to 0 at a distance equal to its focal length. The design operating point will be somewhere in there. Now if you take the spacing past the focal length of the reducer, you get an inverted reduction increasing in magnitude to -1 at double the focal length of the reducer. That's the region I was operating in with my earlier setup. Beyond double the reducer's focal length, you get magnification so that's way out of spec!

As a ball park estimate, the focal length of the focal reducer is around 80mm. This is a rounded approximate since the reducer is quite thick and I'm not really sure where the nodal point of it is. The spacing between this reducer and sensor should therefore be 40mm. So the distance I was using it at is totally wrong. The Canon EF mount is 44mm already, and you're adding perhaps another 70mm by the time you got a standard T-ring and eyepiece adapter in place. If I want to use this, I'll need a low profile T-ring and eyepiece adapters. And it'll still be too long. Still, it might make more sense when I get an imaging device without mirror.

Note the above ignores consideration the reducer will be optimised in optical design for its rated reduction factor, so using it outside that will likely cause problems to some degree. Also it's a bit theoretical and I can't claim with confidence to have got it all correct!

I noticed something earlier today while fiddling with my eyepiece adapter. The inside of the barrel is exactly the right diameter to take a 1.25" filter! This means I can put the filter close to the T-ring. So close in fact, it sticks out camera side. I compared it against an EF-S lens which does the same, and as the distance was similar I decided there was minimal risk of mirror collision and gave it a go.

A quick test of the camera (600D) on scope without reducer. A pigeon happened to be standing on the TV aerial of the houses opposite.

By the time I got the focal reducer in place, the pigeon had flown off though. I thought this didn't look right with a defocus pattern.

This is better. I turned the reducer around. It appears to be direction sensitive. This is with the exposed threads pointing sensor side, as it would be if screwed into an eyepiece or similar.

I thought I'd pick a more defined subject for comparison to get a feeling for the reduction. This is a rooftop without reducer.

Same position with reducer. I estimate the reduction is slightly more than 2x (or, magnification <0.5x). There is only a small gain in field of view before the image circle becomes inadequate to cover the sensor.